Ristretto is the mail API that is used by Columba,
a Java mail client (http://columba.sf.net). It is
available as a stand-alone API, and it provides a
well-tested, slim, and fast Java implementation of
all relevant mail protocols and features. It is
licensed under a MPL/LGPL/GPL tri-license.

gavamail is a POP3/IMAP server proxy for Gmail. It
sets up a proxy server on your localhost that
talks with the Gmail service using the g4j java
library. It opens up a port on your local machine
which you can then connect to from your favorite
email client.

IMAPEngine is an IMAP server that uses a tdbengine
database structure to store all accounts,
messages, and mailboxes. This means it has nearly
no limits in numbers, sizes, and performance, and
it uses indexes to sort/find its data. IMAPEngine
is IMAP4Rev1 compatible and also has built-in
support for the ACL, ID, IDLE, and UIDPLUS
extensions. It is not (yet) an MTA, but it
retrieves incoming messages from different POP3
servers. An simple integrated Web interface makes
account configuration easy.

Openmailadmin is a little administration interface to every complete IMAP mail server daemon. It supports every feature IMAP provides, and fits in most MTA configurations. A key feature is the non-standard, generic administration hierarchy which not only seperates "normal users" from "administrators", but enables the mailserver-master to create instances between them. You will be able to let other users create their own sub-users and thus either share a single mail server between different organizations or project your company's employee structure. It excels with features such as regex addresses and folder ACL management.

Deletemail is a non-interactive tool for removing
mail from one or more IMAP4 mailboxes. It removes
mail older than a configurable number of days, and
by default does not remove mail that is not marked
as seen on the server.

imap-purge is a little tool that automatically
purges mailing list folders in an IMAP account.
With very busy mailing lists such as the Linux
kernel list, folders fill up quickly. The tool was
written for the purpose of cleaning up individual
folders that were restricted by a quota. You can
specify a certain time frame (such as the last
thirty days) or a maximum number of messages for
each folder.

Teach-SA reads mail in designated maildir folders (spam on
the one hand, ham on the other) and feeds them to
Spamassassin for Bayesian learning and submission to
various spam detection schemes. It is good for
implementing any sort of supervised training in addition to
Spamassassin's unsupervised training (also known as
automatic whitelist) while reducing training-related admin
workload to nearly zero. It fits any setup storing mail as
maildir, but could trivially be modified to work with mbox
based systems. It was originally developed so that users
could report spam by moving it to a specific folder in their
own IMAP mailbox.

IMAP Calendar Proxy implements an HTTP proxy that
converts HTTP PUT requests of vcalendar data to
separate messages stored on an IMAP server. It can
be used with Mozilla Sunbird/Calendar to allow
storage of a calendar via IMAP.

Knight's Mail is a Web mail application that uses
XUL to render the client display. Its goal is to
look and feel as close to Mozilla Thunderbird as
possible, while maintaining the positive qualities
of a Web mail client such as keeping settings on
the server and only requiring a (Mozilla-based)
browser to be installed on the client.